The present invention relates to a ceramic support element for NOx traps. More specifically, the present invention relates materials for NOx trap supports which exhibits migration resistance to alkali metal NOx storage components combined with good strength and low coefficient of thermal expansion.
Gasoline direct injection (GDI) engines are becoming increasingly popular due to the promise of increased fuel efficiency. Similarly to conventional engines, the exhaust gas discharged from GDI engines needs to purified of nitrogen oxides (NOx). However, unlike conventional engines which employ three-way catalysts, the GDI engine which is a partial lean burn gasoline engine producing exhaust gas with an excess amount of oxygen, cannot only employ the services of three-way catalysts because these require conditions where the air-fuel ratio is substantially stoichiometric.
NOx traps appear to be a leading candidate for exhaust purification in GDI engines. NOx traps are similar to three-way catalysts, in that they are made of a support and a catalyst coating the support, with the difference existing in that NOx traps include an additional component in the catalyst coating which stores the trapped NOx. The NOx storage component is typically an alkaline earth, alkali, or rare-earth metal compounds. Alkali metal-based NOx traps hold the most promise in providing high trapping efficiency over a wide temperature range. A disadvantage of alkali metal-based NOx support components, however, is incompatibility with cordierite supports, the standard in the three-way catalyst industry. Research has shown that the alkali metal in the catalyst coating migrates into the cordierite support during use at high temperatures. This results in a decrease in the trapping efficiency of the NOx trap and alteration of the chemical composition of the underlying material destroying the desirable properties of the NOx trap support.
Recently, much effort has been concentrated in the area of developing NOx support materials that resist migration of NOx alkali metal storage components, and therefore do not suffer the deficiencies associated with cordierite. Co-pending provisional application entitled xe2x80x9cCatalyst for Purifying Exhaust Gasesxe2x80x9d by Cutler et al., co-assigned to the present assignee, and herein incorporated by reference in its entirety is directed at such materials. Such materials, however, are expected to exhibit a low four-point modulus of rupture strength of less than 1000 pounds per square inch (psi) as measured on a solid rod of circular cross section, which is undesirable for applications in harsh environments.
A need therefore exists to have NOx trap support materials which resist migration of NOx alkali metal storage components with higher strength than has been achieved up to now, while at the same time with low coefficient of thermal expansion.
The present invention provides novel materials for NOx trap supports, the materials offering resistance to migration of NOx alkali metal storage components in combination with high strength and low coefficient of thermal expansion.
These novel materials are selected from the ternary systems of Al2TiO5xe2x80x94MgTi2O5xe2x80x94MgAl2O4 and Al2TiO5xe2x80x94FeTiO5xe2x80x94Al2O3. In a preferred embodiment the inventive materials consist essentially, in terms of weight percent on an oxide basis, of: 15 to 90 wt. % Al2TiO5, 2 to 45 wt. % MgTi2O5, and 5 to 55 wt. % MgAl2O4. In another preferred embodiment the inventive materials consists essentially, in terms of weight percent on an oxide basis, of: 35 to 85 wt. % Al2TiO5, 2 to 35 wt. % FeTiO5, and 5 to 35 wt. % Al2O3.
The inventive NOx support materials combine properties of good strength and low coefficient of thermal expansion with resistance to migration of alkali metal from the NOx storage component. The coefficient of thermal expansion (22-800xc2x0 C.) is less than 20xc3x9710xe2x88x927/xc2x0 C., preferably less than 10xc3x9710xe2x88x927/xc2x0 C., and the modulus of rupture as measured on a solid rod of circular cross section is greater than 1000 pounds per square inch, and preferably greater than 2000 pounds per square inch.
The following are definitions of terms useful in understanding the present invention:
xe2x80x9cNOx trapxe2x80x9d means a device capable of storing and reducing nitrogen oxides (NOx) under alternating lean-burn and rich-burn conditions. A NOx trap is composed of a support element for loading a NOx catalyst coating containing a NOx storage component;
xe2x80x9cNOx support elementxe2x80x9d means a body onto which a NOx catalyst coating containing a NOx storage component is loaded;
xe2x80x9cNOx catalyst coatingxe2x80x9d is composed of a carrier material, a NOx storage component, and a noble metal catalyst;
xe2x80x9cNOx storage componentxe2x80x9d functions to adsorb/store the nitrogen oxides and contains at least an alkali metal;
xe2x80x9cLean-burn environmentxe2x80x9d occurs when there is an excess amount of oxygen;
xe2x80x9cRich-burn environmentxe2x80x9d occurs when there are fuel-rich conditions.